DEVELOPMENT OF THE TWO MIDDLE GERAI-LAYERS. 121 



first the changes which may be recognised in viewing the germ-disc 

 from the surface, and to these shall add, secondly, the more im- 

 portant results acquired by series of cross sections. 



At the posterior margin of the germ-disc of the Chick (fig. 81 A), 

 which consists of two layers lying on the yolk like a watch-glass, we 

 had distinguished not only a short time before incubation, but also 

 during the early hours of that process the crescent (s) and the 

 crescentic groove, and had learned to recognise that this was the 

 place from which the inner germ-layer arose by a process of folding 

 under. 



When, during the first hours of incubation, the germ-layers grow 

 out farther on the yolk, the crescentic groove (fig. 81 .Z?) is con- 

 verted into the primitive groove (pr), a structure of far-reaching 

 significance. 



The metamorphosis, according to the excellent researches of DUVAL, 

 takes place in the following manner : In the middle of the anterior 

 blastoporic lip, where the outer germ-layer bends over to become 

 continuous with the inner, there arises a small notch, which is 

 directed forwards (fig. 81 A sk) ; this gradually elongates into a 

 groove (fig. 81 .5), corresponding with the future longitudinal axis of 

 the embryo, and by the following method : the right and the left halves 

 of the [anterior] blastoporic lip, together with the part which bounds 

 the first notch, grow toward each other, and come in contact with 

 each other in the median plane, with the same rapidity with which the 



disc increases in super- 



A B c 



ficial extent. For a time, ,. .. 



therefore, the blastopore ,.-;;~~~-^ /.'''--"~,~,~^\*\ 



has the form of a short '/~'"~'\\ ',''/'* " V'. \ : :' ;'' "' ' : '; 



longitudinal groove, i( \\'\ / V\Vv. ,//'/'.' 



which, at its posterior N^^' '*^~-&:'-' 



end, is beilt around into pig. 82. Diagrams to elucidate the formation of theprimi- 

 two Short transversely tive groove, after DUVAL. 



The increasing size of the germ-disc in the course of the 



placed crescentic horns development is indicated by dotted circular lines. The 



(g). Finally these also heay y lines ^resent the crescentic groove, and the 



primitive groove which arises from it by the fusion of 

 have disappeared ; they, the edges of the crescent. 



too. have grow y n toward 



each other, toward the median plane, and have thus contributed 

 largely to the posterior elongation of the primitive groove. By this 

 remarkable process of growth the whole blastopore is converted from 

 a ti ansverse fissure into a longitudinal one. 



The accompanying diagrams (fig. 82) serve to illustrate this highly 



